Mixed-metal oxide absorbent for carbon dioxide and water removal

Abstract

Mixed-metal oxide absorbents having superior CO 2 and water removal capacity and superior regenerability are prepared from novel mixed-metal carbonate precursors or from mixed-metal oxides derived from those precursors. The precursors are prepared from a salt of silver and the salt of a second metal. The second metal may be selected from the group consisting of magnesium, iron, cobalt, nickel, zinc, and other metals for which the metal carbonate to metal oxide reaction occurs at a temperature of not greater than about 400° C. The precursors have an unique x-ray diffraction pattern characteristic of the silver carbonate pattern rather than of the physical mixture of the silver carbonate and the carbonate of the second metal.

I claim: 
     
       1. A mixed-metal carbonate precursor comprising silver and zinc and having a characteristic x-ray powder diffraction pattern which contains at least the d-spacing set forth in Table VII. 
     
     
       2. The precursor of claim 1 wherein the second metal is zinc. 
     
     
       3. The precursor of claim wherein the atomic ratio of Ag 2  to the second metal is about 0.1 to 10. 
     
     
       4. The precursor of claim 1 in which the ratio of Ag 2  to the second metal is controlled by the ratio of Ag 2  to the second metal in the first solution. 
     
     
       5. A mixed-metal oxide prepared by the process comprising the steps of: a) activating the mixed-metal carbonate precursor of claim 1 in air having a relative humidity ranging from 5 to 85%, at a temperature from about 200°-300° C., and at a pressure of about 50-101 kPa until the CO 2  concentration has decreased to less than about 0.02 volume percent; and   b) cooling said activated precursor to room temperature in humid air to form said oxide.   
     
     
       6. A mixed-metal oxide absorbent prepared by the process comprising the steps of: a) mixing the mixed-metal oxide of claim 5 with a binder and promoter to form a mixture;   b) adding sufficient water to said mixture to obtain a paste;   c) extruding said paste to form an extrudate; and   d) drying said extrudate in air to form said absorbent.   
     
     
       7. A process for preparing a mixed-metal oxide absorbent comprising the steps of: a) mixing the mixed-metal oxide of claim 5 with a binder and a promoter to form a mixture;   b) adding sufficient water to said mixture to obtain a paste;   c) extruding said paste to form an extrudate; and,   d) drying said extrudate in air to form said absorbent.   
     
     
       8. A mixed-metal oxide absorbent prepared by the process comprising the steps of: a) mixing the mixed-metal carbonate precursor of claim 1 with a binder and a promoter to form a precursor mixture;   b) adding sufficient water to said precursor mixture to obtain a precursor paste;   c) extruding said precursor paste to form an extrudate;   d) drying said extrudate in air;   e) activating said dried extrudate in air having a relative humidity ranging from 5 to 85%, at a temperature from about 200°-300° C. at a pressure of from about 50-120 kPa until the CO 2  concentration decreases to less than about 0.02 volume percent; and,   f) cooling said activated extrudate to room temperature in air having a relative humidity ranging from 5 to 85% to form said absorbent.   
     
     
       9. The absorbent of claim 8 wherein the second metal is zinc. 
     
     
       10. The absorbent of claim 8 wherein the binder comprises an inorganic compound selected from the group consisting of sodium silicate, alumina, silica, and clay, or an organic compound selected from the group consisting of cellulose acetate, and polyethylene imine. 
     
     
       11. The absorbent of claim 8 wherein the promoter comprises a material which will promote CO 2  absorption on the mixed-metal oxide absorbent. 
     
     
       12. The absorbent of claim 8 wherein the promoter comprises KOH, NaOH, Mg(OH) 2 , and other hydroxides of alkali or alkaline earth metals. 
     
     
       13. The absorbent of claim 8 in which the mixture contains about 3-25% by weight of binder and about 5-30% by weight of promoter with the balance comprising the oxide or precursor. 
     
     
       14. A process for removing both carbon dioxide and water from air comprising passing a stream of air over the mixed-metal oxide absorbent of claim 8. 
     
     
       15. A process for preparing a mixed-metal oxide comprising the steps of: a) activating the mixed-metal carbonate precursor of claim 1 in air having a relative humidity ranging from 5 to 85%, at a temperature from about 200°-300° C., and at a pressure of about 50-101 kPa until the CO 2  concentration has decreased to less than about 0.02 volume percent; and   b) cooling said activated precursor to room temperature in humid air.   
     
     
       16. A process for preparing a mixed-metal oxide absorbent comprising the steps of: a) mixing the mixed-metal carbonate precursor of claim 1 with a binder and a promoter to form a precursor mixture;   b) adding sufficient water to said precursor mixture to obtain a precursor paste;   c) extruding said precursor paste to form an extrudate;   d) drying said extrudate in air;   e) activating said dried extrudate in air having a relative humidity ranging from 5 to 85%, at a temperature from about 200°-300° C. at a pressure of from about 50-120 kPa until the CO 2  concentration decreases to less than about 0.02 volume percent; and,   f) cooling said activated extrudate to room temperature in air having a relative humidity ranging from 5 to 85% to form said absorbent.   
     
     
       17. A mixed-metal carbonate precursor prepared by the process comprising the steps of: a) mixing a first solution comprising a salt of silver and a salt of a second metal with a second solution comprising a carbonate or bicarbonate compound, wherein the salt of the second metal is selected from the group consisting of magnesium, iron, cobalt, nickel, zinc, and other metals for which the metal carbonate to metal oxide reaction occurs at a temperature of not greater than about 400° C., and wherein both first and second solutions have concentrations of from about 0.01 to 0.5 N;   b) storing the resultant mixture of solutions at a temperature of from about 0° to 10° C. for about 12 and 48 hours;   c) recovering the fine crystallites from said mixture of solutions by vacuum filtration; and,   d) drying said crystallites in a vacuum oven to produce said precursor.   
     
     
       18. The precursor of claim 17 wherein the second metal is zinc. 
     
     
       19. The precursor of claim 17 wherein the atomic ratio of Ag 2  to the second metal is about 0.1 to 10. 
     
     
       20. The precursor of claim 17 in which the ratio of Ag 2  to the second metal is controlled by the ratio of Ag 2  to the second metal in the first solution. 
     
     
       21. A mixed-metal oxide prepared by the process comprising the steps of: a) activating the mixed-metal carbonate precursor of claim 17 in air having a relative humidity ranging from 5 to 85%, at a temperature from about 200°-300° C., and at a pressure of about 50-101 kPa until the CO 2  concentration has decreased to less than about 0.02 volume percent; and   b) cooling said activated precursor to room temperature in humid air to form said oxide.   
     
     
       22. The mixed-metal oxide of claim 21 wherein the second metal is zinc. 
     
     
       23. A mixed-metal oxide absorbent prepared by the process comprising the steps of: a) mixing the mixed-metal oxide of claim 21 with a binder and a promoter to form a mixture;   b) adding sufficient water to said mixture to obtain a paste;   c) extruding said paste to form an extrudate; and,   d) drying said extrudate in air to form said absorbent.   
     
     
       24. A mixed-metal oxide absorbent prepared by the process comprising the steps of: a) mixing the mixed-metal carbonate precursor of claim 17 with a binder and a promoter to form a precursor mixture;   b) adding sufficient water to said precursor mixture to obtain a precursor paste;   c) extruding said precursor paste to form an extrudate;   d) drying said extrudate in air;   e) activating said dried extrudate in air having a relative humidity ranging from 5 to 85%, at a temperature from about 200°-300° C. at a pressure of from about 50-120 kPa until the CO 2  concentration decreases to less than about 0.02 volume percent; and,   f) cooling said activated extrudate to room temperature in air having a relative humidity ranging from 5 to 85% to form said absorbent.   
     
     
       25. The absorbent of claim 24 the second metal is zinc. 
     
     
       26. The absorbent of claim 24 wherein the binder comprises an inorganic compound selected from the group consisting of sodium silicate, alumina, silica, and clay, or an organic compound selected from the group consisting of cellulose acetate, and polyethylene imine. 
     
     
       27. The absorbent of claim 24 wherein the promoter comprises a material which will promote CO 2  absorption on the mixed-metal oxide absorbent. 
     
     
       28. The absorbent of claim 24 wherein the promoter comprises KOH, NaOH, Mg(OH) 2 , and other hydroxides of alkali or alkaline earth metals. 
     
     
       29. The absorbent of claim 24 in which the mixture contains about 3-25% by weight of binder and about 5-30% by weight of promoter with the balance comprising the oxide or precursor. 
     
     
       30. A process for removing both carbon dioxide and water from air comprising passing a stream of air over the mixed-metal oxide absorbent of claim 24. 
     
     
       31. A process for preparing a mixed-metal oxide comprising the steps of: a) activating the mixed-metal carbonate precursor of claim 17 in air having a relative humidity ranging from 5 to 85%, at a temperature from about 200°-300° C., and at a pressure of about 50-101 kPa until the CO 2  concentration has decreased to less than about 0.02 volume percent; and   b) cooling said activated precursor to room temperature in humid air.   
     
     
       32. A process for preparing a mixed-metal oxide absorbent comprising the steps of: a) mixing the mixed-metal carbonate precursor of claim 17 with a binder and a promoter to form a precursor mixture;   b) adding sufficient water to said precursor mixture to obtain a precursor paste;   c) extruding said precursor paste to form an extrudate;   d) drying said extrudate in air;   e) activating said dried extrudate in air having a relative humidity ranging from 5 to 85%, at a temperature from about 200°-300° C. at a pressure of from about 50-120 kPa until the CO 2  concentration decreases to less than about 0.02 volume percent; and,   f) cooling said activated extrudate to room temperature in air having a relative humidity ranging from 5 to 85% to form said absorbent.   
     
     
       33. A process for preparing a mixed-metal carbonate precursor comprising the steps of: a) mixing a first solution comprising a salt of silver and a salt of a second metal with a second solution comprising a carbonate or bicarbonate compound, wherein the salt of the second metal is selected from the group consisting of magnesium, iron, cobalt, nickel, zinc, and other metals for which the metal carbonate to metal oxide reaction occurs at a temperature of not greater than about 400° C., and wherein both first and second solutions have concentrations of from about 0.01 to 0.5 N;   b) storing the resultant mixture of solutions at a temperature of from about 0° to 10° C. for about 12 and 48 hours;   c) recovering the crystallites from said mixture of solutions by vacuum filtration; and,   d) drying said crystallites in a vacuum oven to produce said precursor.